The complement system is a complex enzyme cascade made up of a series of serum glycoproteins, that normally exist in inactive, pro-enzyme form. Three main pathways, the classical and the alternative pathway, can activate complement, which merge at the level of C3, where two similar C3 convertases cleave C3 into C3a and C3b. An additional pathway, the mannose-binding lectin (MBL) pathway has also been described.
Classical pathway components are labeled with a C and a number (e.g. C1, C3). Because of the sequence in which they were identified, the first four components are numbered C1, C4, C2, and C3. Alternative pathway components are lettered (e.g. B, P, D). Cleavage fragments are designated with a small letter following the designation of the component (e.g. C3a and C3b are fragments of C3). Inactive C3b is designated iC3b. Polypeptide chains of complement proteins are designated with a Greek letter after the component (eg, C3α and C3β are the α- and β-chains of C3). Cell membrane receptors for C3 are abbreviated CR1, CR2, CR3, and CR4.
The classical pathway of the complement system is a major effector of the humoral branch of the human immune response. The trigger activating the classical and MBL pathways is either IgG or IgM antibody bound to antigens or lectin on the target cells. Binding of antibody to antigen exposes a site on the antibody which is a binding site for the first complement component, C1. C1 binds to the exposed regions of at least two antigen-bound antibodies, and as a result, its C1r and C1s subunits are activated. Activated C1s is responsible for the cleavage of the next two involved complement components, C4 and C2. C4 is cleaved into two fragments, of which the larger C4b molecule attaches to the target membrane nearby while the small C4a molecule leaves. An exposed site on deposited C4b is available to interact with the next complement component, C2. Just as in the previous step, activated C1s cleaves the C2 molecule into two pieces, of which the fragment C2a remains, while the smaller C2b fragment leaves. C4b2a, also known as the C3 convertase, remains bound to the membrane. This C3 convertase converts the next complement component, C3 into its active form.
Activation of the alternative complement pathway begins when C3b binds to the cell wall and other cell components of the pathogens and/or to IgG antibodies. Factor B then combines with cell-bound C3b and forms C3bB. C3bB is then split into Bb and Ba by factor B, to forming the alternative pathway C3 convertase, C3bBb. Properdin, a serum protein, then binds C3bBb and forms C3bBbP that functions as a C3 convertase, which enzymatically splits C3 molecules into C3a and C3b. At this point, the alternative complement pathway is activated. Some of C3b binds to C3bBb to form C3bBb3b, which is capable of splitting C5 molecules into C5a and C5b.
The alternative pathway is a self-amplifying pathway and is important in the clearance and recognition of bacteria and other pathogens in the absence of antibodies. The alternative pathway can also amplify complement activation after initial complement activation by either the lectin and/or classical pathway. The rate-limiting step of activation of the alternative pathway in humans is the enzymatic action of factor D on the cleavage of factor B to form the alternative pathway C3 convertase, C3bBb. (Stahl et al., American Journal of Pathology 162:449-455 (2003)). There is strong evidence for the role of complement activation and deposition in adjuvant-induced arthritis (AIA), and collagen-induced arthritis (CIA) and in a variety of other diseases and conditions.
Recently, defective alternative pathway control has been implicated in the development of kidney and eye diseases, including hemolytic uremic syndrome (HUS) and AMD (Zipfel et al., Mol. Immunol. 43:97-106 (2006), available online at www.sciencedirect.com). C3 has been found to be essential for the development of CNV in mice (Bora et al., J. Immunol. 174(l):491-7 (2005)).
The role of the complement system in inflammatory conditions and associated tissue damage, autoimmune diseases, and complement-associated diseases is also well known.
It has been suggested that the alternative pathway plays an important role in inflammation (Mollnes et al., Trends in Immunology 23:61-64 (2002)), local and remote tissue injury after ischemia and reperfusion (Stahl et al., supra); adult respiratory distress syndrome (ARDS, Schein et al., Chest 91:850-854 (1987)); complement activation during cardiopulmonary bypass surgery (Fung et al, J Thorac Cardiovasc Surg 122:113-122 (2001)); dermatomyositis (Kissel, J T et al, NEJM 314:329-334 (1986)); and pemphigus (Honguchi et al, J Invest Dermatol 92:588-592 (1989)). The alternative complement pathway has also been implicated in autoimmune diseases, such as, for example, lupus nephritis and resultant glomerulonephritis and vasculitis (see, e.g. Watanabe et al., J. Immunol. 164:786-794 (2000)); and rheumatoid arthritis, such as juvenile rheumatoid arthritis (Aggarwal et al., Rheumatology 29:189-192 (2000); and Neumann E. et al, Arthritis Rheum. 4:934-45 (2002)).
Local increase in complement deposition and activation correlate with disease severity (Atkinson, J Clin Invest 112:1639-1641 (2003)). C5a receptor antagonists, such as peptides and small organic molecules, have been tested for the treatment of arthritis (Woodruf et al., Arthritis & Rheumatism 46(9):2476-2485 (2002)), and various other immunoinflammatory diseases (Short et al., Br J Pharmacol 126:551-554 (1999); Finch et al., J Med Chem 42:1965-1074 (1999)); and companies, such as Promics (Australia) have been conducting human clinical trials to test the efficacy of C5a antagonists in similar indications. C5a has also been implicated in dermatomyositis, and pemphigus. (Kissel, J T et al, NEJM 314:329-334 (1986)). Anti-C5a monoclonal antibodies have been shown to reduce cardiopulmonary bypass and cardioplegia-induced coronary endothelial dysfunction (Tofukuji et al., J. Thorac. Cardiovasc. Surg. 116:1060-1069 (1998)), prevent collagen-induced arthritis and ameliorate established disease (Wang et al., Proc. Natl. Acad. Sci. USA 92(19):8955-8959 (1995)).
Opsonophagocytosis, the process of deposition of complement fragments on the surface of particles and the subsequent uptake by phagocytic cells, is crucial for the clearance of circulating particles including immune complexes, apoptotic cells or cell debris and pathogens (Gasque, P., Mol Immunol. 41:1089-1098 (2004)). Tissue resident macrophages are known to play an important role in the complement mediated clearance of particles from the circulation. Kupffer cells, constituting over 90% of the tissue resident macrophages, are continuously exposed to blood from the hepatic portal vein and are strategically positioned in liver sinusoids to efficiently clear opsonized viruses, tumor cells, bacteria, fungi, parasites and noxious substances from the gastrointestinal tract. This clearance process is for a large part dependent on the presence of complement C3 as an opsonin (Fujita et al., Immunol. Rev. 198:185-202 (2004)). Upon binding to bacterial surfaces via a thoesther, C3 is cleaved and amplifies the alternative pathway of complement. This reaction leads to further deposition of C3 fragments that can serve as ligands for complement receptors on macrophages. The importance of this pathway is shown by the high susceptibility of humans lacking C3 to bacterial and viral infections (ref).
The complement receptors characterized so far, CR1, 3 and 4 internalize C3b and phagocytosis C3 opsonized particles only after PKC activation or Fc receptor stimulation (Carpentier et al., Cell Regul 2, 41-55 (1991); Sengelov, Crit.Rev.Immunol. 15: 107-131 (1995); Sengelov et al., J. Immunol. 153:804-810 (1994). Moreover, CR1 is not expressed on the surface of murine Kupffer cells (Fang et al., J. Immunol. 160:5273-5279 (1998) Complement receptors that aid KCs in the constitutive clearance of circulating particles have not been described so far.
An anti-C3b(i) antibody has been reported to enhance complement activation, C3b(i) deposition, and killing of CD20+ cells by rituximab (Kennedy et al., Blood 101(3):1071-1079 (2003)).
In view of the known involvement of the complement cascade in a variety of diseases, there is a need for identification and development of new pharmaceuticals for the prevention and/or treatment of complement-associated diseases.